Biaxially stretched polyester films are widely used as materials for magnetic recording media, electronic appliances, etc. because of their excellent mechanical properties, electrical properties and chemical resistance. However, they have a resistivity of 10.sup.15-10.sup.16 .OMEGA./.quadrature. and are easily charged with electricity. Therefore, they are susceptible to troubles caused by adherence of dust and other foreign materials when they are used as magnetic recording media or materials for electronic appliances or tapes therefor and suffer troubles when tapes thereof travel on appliances.
For the prevention of electrostatic charging of polyester films, it is conventional to apply an ionic-conductive antistatic material such as a surfactant or an electronic-conductive antistatic material such as carbon black to the film surface or to incorporate an antistatic material into the base polymer.
For economically producing conductive films having non-humidity-dependent antistaticity, there have been proposed methods, which comprise applying an electronic-conductive fine powder such as carbon black by the application-drawing method in the course of manufacturing polyester films as disclosed in Japanese Laid-Open Patent Publication Nos. 1-267830 and 2-67335.
When a coating material containing a large amount of conductive fine powder is used in the application-drawing method, however, the coating layer suffers minute cracking and a smooth surface suitable for magnetic recording media or materials for electronic appliances cannot be obtained. Therefore, there has been a need for improvement of polyester films in this respect.
We conducted an intensive study regarding the above-described need, we have found that a flat conductive film having excellent light-shielding property can be obtained by applying a coating composition having specific properties to the surface of base films by the application-drawing method.